Recently, liquid crystals have been investigated, besides in application to display media based on reversible movement of liquid crystal molecules such as display elements representatively exemplified by TN-type and STN-type liquid crystals, towards application to optically anisotropic materials for retardation plates, polarization plates, polarizing prisms, various optical filters, and others due to their oriented nature and anisotropy in physical properties such as refractive index, dielectric constant, and magnetic susceptibility.
In order to provide stable and uniform optical properties to such optically anisotropic materials using liquid crystal substances as source components, it is essential to form mechanically and thermally stable polymers with high glass transition temperature (referred to as “Tg” hereinafter) and excellent orientation by semipermanently fixing uniform orientation of the liquid crystal molecules in a liquid crystal state.
As a strategy to semipermanently fix uniform orientation of liquid crystal molecules in a liquid crystal state, it is already known, for example, a method of uniformly orienting a liquid crystalline compound with a polymerizable functional group or a polymerizable liquid crystal composition containing such a compound in its liquid crystal state, followed by photo-polymerizing by irradiation with energy beam such as ultraviolet light while keeping the liquid crystal state to semipermanently fix the is uniform orientation.
As liquid crystalline compounds with polymerizable functional group, (meth)acrylic group-containing polymerizable compounds are known (for example, see Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5). The temperature at which the polymerizable compounds described in these documents exist in a liquid crystal phase was high, and hence the polymerization-curing had to be conducted at high temperatures.
Since polymerization at a high temperature causes thermal polymerization concurrently with photo-polymerization, the cured products tend to exhibit uneven physical properties, and it is difficult to fix the orientation. Also, for easier temperature control in curing, there has been awaited compounds that provide liquid crystal compositions existing in a liquid crystal phase at lower temperatures.
Patent Document 1: Japanese Patent Laid-Open Publication H9-40585
Patent Document 2: Japanese Patent Laid-Open Publication H11-116534
Patent Document 3: Japanese Patent Laid-Open Publication H11-513360
Patent Document 4: Japanese Patent Laid-Open Publication 2002-145830
Patent Document 5: Japanese Patent Laid-Open Publication 2004-041129